What is usury?
Usury is an act of injustice which consists in taking advantage of the need of our neighbour by lending him money, or any other thing that has a money value (whose only use is the consumption thereof which is destined to meet present necessities), and in return obliging him to give back the money or the thing lent by a fixed date with an addition as the price of the use (LXXVIII. 1, 2, 3). [Contra Vix Pervenit 3.I]
Is usury the same thing as lending out at interest?
No; for although all usury is lending out at interest, all lending out at interest is not usury.
In what does lending out at interest differ from usury?
It differs from usury in so far as money is considered as productive, by reason of the social and economic circumstances in which we live to-day.
What is necessary in order that lending out at interest may be allowable and may not run the risk of becoming usury?
Two things are necessary: first, the amount of interest charged must not exceed the legal charge, or the charge fixed by reasonable custom; and second, those who are well off should not be exacting towards the poor who have need of borrowing, not in order to trade in money but with the object of immediate consumption and the succouring of their needs. [Contra Vix Pervenit 3.II]
(emphasis is mine)
It is curious to wonder when and whence these error arose. Usury is charging excessive interest against the poor. Usury is done away with when considering money qua productive. It is particularly strange that a member of the Order of Preachers writing a catechism based on the Summa Theologica would get usury so wrong with little relation between what he and Aquinas wrote.